Recently, methods of fabricating nitride gallium (GaN)-based white light emitting diodes (LED), which have actually been researched and studied all over the world, are mainly classified into two methods, in which one method is to acquire a white color by combining a phosphorous material on a blue LED chip or a UV LED chip in the form of a single chip, and the other method is to acquire the white color by combining two or three LED chips with each other in the form of a multiple-chip.
A representative method of realizing a white LED through the form of the multiple-chip is to combine three RGB chips with each other. According to the method, the operating voltage is irregularly represented in each chip, and the output of each chip varies according to the surrounding temperatures so that a color coordinate is changed.
Due to the above problem, the form of the multiple-chip is suitable for a special lighting field of requiring the realization of various colors by adjusting the intensity of each LED through a circuit structure instead of the realization of the white LED.
Accordingly, in order to realize the white LED, a binary system, in which a blue LED easily fabricated and representing superior efficiency and a phosphor pumped by the blue LED to emit yellow light are combined with each other, has been representatively used.
The binary system mainly employs a white LED realized by using the blue LED as a pumping light source and by pumping a yttrium aluminum garnet (YAG) phosphor, which is activated by Ce3+ (which is a trivalent rare earth element), that is, a YAG:Ce phosphor by the light output from the blue LED.
In addition, the white LED is packaged and used in various forms according to the application fields thereof. Representatively, the white LED is manly used in an ultra micro-size LED device having the type of a surface mounting device (SMD) applicable to backlighting of a cellular phone and a vertical lamp type LED device for an electric board and a solid state display device or an image display.
Meanwhile, indexes used to analyze the characteristic of white light include a correlated color temperature (CCT) and a color rendering index (CRI).
The CCT refers to the temperature of a black body on the assumption the temperature of the black body matches with the temperature of an object when the color of a visible ray emitted from the object seems to be identical to the color radiated from the black body. As the color temperature is increased, a dazzling bluish white color is represented.
In other words, white light having a low color temperature is expressed warmly, and white light having a high color temperature is expressed coldly. Therefore, the white light can satisfy even the characteristic of a special lighting field requiring various colors by adjusting color temperatures.
According to the related art, the white LED using the YAG:Ce phosphor represents only a color temperature of 6000K to 8000K. In addition, the CRI represents the color difference of an object when the sunlight is irradiated onto the object and when other artificial lighting is irradiated onto the object. When the color of the object is identical to the color of the sunlight, the CRI is defined as 100. In other words, the CRI is an index representing the approximation degree of the color of the object under artificial lighting to the color of the object under the sunlight, and has a numeric value of 0 to 100.
In other words, a white light source having a CRI approximating 100 provides a color of an object approximately matching with a color of the object perceived by a human eye under the sunlight.
Recently, when comparing with an incandescent lamp having the CRI of over 80, and the fluorescent lamp having the CRI of over 75, a commercialized white LED represents the CRI of about 70 to about 75.
Therefore, the white LED employing the YAG:Ce phosphor according to the related art represents a relatively low CCT and a relatively low CRI.
In addition, since only the YAG:Ce phosphor is used, the adjustment of the color coordinate, the CCT, and the CRI may be difficult.
In relation to a light emitting diode employing a phosphor as described above, Korean Unexamined Patent Publication No. 10-2005-0098462 has been published.